It prevents most bots from getting through, at least automated bots that seek out empty input boxes and add random stuff to them and attempt to hit a submit button. I had tried Google Recaptcha but there were just too many issues with it, for whatever reason, so I had created my own. I'm still experimenting with several different measures for improving the captcha. I've had two attacks since its creation 2 years ago. The spammer was able to create over 5,000 posts in just a few hours.
The first spammer actually emailed me and was pissed off. The entire email was in Russian but it translated into: "Why did you delete all of my posts I created?" Definitely correlated as it was just a few hours after I deleted them that I received that email.
The most recent attack was just a few weeks ago. I added some additional "captures" which seems to have prevented anything further. These pages were also in Russian and basically just linked to random forums all over the Internet. I don't think MyPost has any potential to give "rank" at all, but I'm pretty sure it's being done for backlinks. It may be that the "spammer" is a live person that goes through the source code and actually writes their own code to break through and create posts on MyPost.
The only thing I can keep doing for the time being is learning their methods and implementing new measures to prevent them from getting through, just as they continue to learn mine. There was a Show HN a few days ago that actually encrypted passwords on the client side, so I may look into that code and revise it to work as a captcha.